A display device typically has an ON state and an OFF state wherein the display device is able to display information only when power is being continuously applied to the display device. For example, most televisions operate in this manner. Thus, these display devices have a single stable state in which the state of the display device is maintained without additional power being applied to the display device and the single stable state in the OFF state. The ON state is thus unstable since the ON state is only maintained by applying power to the display device. These typical display devices consume a significant amount of power since power must be applied to the display device to display an image or text.
Bistable display devices also exist that can, to some extent, maintain the ON state (and continue to display images/text) with reduced power consumption or no power consumption for an amount of time. Thus, the OFF state and the ON state are stable states because both states can be maintained without the application of a significant amount of additional power. There are a few different types of bistable displays, such as the electrophoretic displays, electrochromic displays, electrowetting displays, MEMS displays, and LCD displays. For the known bistable LCD displays, there are a few types including nematic types (ZBD and BiNem); cholesteric types (BCD and PDChLC) and smectic types (SSFLC and Smectic-A LCD).
FIG. 1 illustrates an example of the structure of a liquid crystal display 10. The display 10 has a first and second substrate 12, 14 (which typically may be made of glass) that contain and sandwich liquid crystal material 16. One or more of the substrates may also contain an electrode (often made of indium tin oxide since it is transparent to visible light) that provides power to the display and induces an electric field that is used to control the liquid crystal material and therefore display information. The display 10 may also have a layer 18 that absorbs light that passes through the liquid crystal in the ON state and produces a better black state in which the display is displaying information.
A cholesteric type liquid crystal is one of three major liquid crystal phases. In the cholesteric liquid crystal, the planar texture and the focal-conic texture are two stable textures. In the planar texture, the liquid crystal molecules are basically aligned to be parallel to the substrates. In other words, the helical axes of the cholesterics are perpendicular to the substrates. In the focal-conic texture, the helical axes of many cholesteric liquid crystal domains are randomly oriented with no preferred direction.
One type of cholesteric type liquid crystal display is a positive cholesteric bistable display. For the positive liquid crystal with the positive dielectric anisotropy, a high voltage is able to unwind the helix of cholesteric liquid crystal and turns them into a homeotropic state to orient along the electric field. Releasing the field, the liquid crystal will relax into either the planar texture or the focal conic texture, depending on how the post-field is applied. At a low post-voltage, the liquid crystal will end up at the focal conic texture. If the post-voltage is relatively high, the planar texture will be reached. In this device, both surfaces are homogeneously aligned but usually with no preferred azimuthal orientations.
The cholesteric liquid crystal may be tuned by the chiral dopants to have a particular intrinsic reflective spectra within the visible band. At the planar texture, a colored light is reflected back due to the so-called Bragg-like scattering. While in the focal conic texture, the incident light basically passes through the thin layer liquid crystal to reach aback layer behind the glass and hence the device takes on the color of that layer in that region.
Another type of conventional LCD display is a negative cholesteric LCD device. An enhancement to that device is a polymer-enhanced negative cholesteric liquid crystal (PENChLC). It works in the transmission mode and is basically a one-pixel panel. PENChLC utilizes polymer networks to establish bistability in negative cholesteric based displays. The ionic compounds are intentionally blended in the host liquid crystal solution to increase its conductivity. Thus, the polymer networks are built inside the liquid crystals to confine the mobility of liquid crystal molecules in order to establish the long term bistability. While these devices provide long term bistability, they are more difficult to manufacture and more expensive since the polymer network has to be added into the liquid crystal material. In addition, liquid crystal device fabrication lines prefer to not manufacture these devices with the polymer network since adding the polymer network is messy and introduces additional step(s) into the fabrication process so that the fabrication lines charge a premium to manufacture these devices which further increases the cost of the polymer network LCDs.
Thus, it is desirable to provide a lower power bi-stable device that provides long term bistability, but does not use polymer networks and it is to this end that the device and method are directed.